In order to implement acyclic communication in an Ethernet network, for example in an EtherCAT network according to IEC Standard “IEC 61158” in such a manner that the cyclic real-time communication is not disturbed, there are usually two options. In option 1, a master controls the complete acyclic communication, that is to say that the acyclic messages which must be transmitted from a slave to the master, are also fetched from the master (for example in an EtherCAT communication network known from the prior art). In the second option, phases are specified in which acyclic communication is allowed to take place (for example in a PROFINET IRT communication network also known from the prior art). In option 1, the line cannot be used in both directions during the transmission of the acyclic messages since either an acyclic message is transmitted to the slave (write, the returning message is without content in this case), or read by the slave (read, the incoming message is unused in this case). In option 2, overload situations can easily occur since a number of slaves have to send messages at the same time. In addition, the configuration of the various phases is complex.
In cyclic communication, the data transmission is performed in iterative, equally long time intervals, the cycles. Cyclic communication occurs usually in each cycle again and again in the same manner, for example for the exchange of process data (inputs/outputs) but also for status monitoring. The exchanged volume of data between individual subscribers is in each case rather low; as a rule, real-time requirements are present.
Acyclic communication, as a rule, does not occur in each cycle but usually only as required. On the other hand, the volume of data is frequently greater than in the case of process data; the transmission has no or only few real-time requirements. The greater volume of data must frequently be transmitted in relatively small units between the cyclic data in order not to impair their real-time requirements.